Engines produce a number of exhaust gases, some of which are considered environmentally undesirable. One of these by-products is NO.sub.x compounds. NO.sub.x compounds are generated during the combustion of hydrocarbon fuels, especially at increased combustion temperatures.
The production of harmful NOx compounds may be reduced by lowering the combustion temperature. One method for lowering the combustion temperature is to route exhaust gas after combustion back into the combustion chamber for mixing with the next air and fuel charge. This exhaust gas displaces some air and fuel charge, thus reducing the heat generated during combustion and thus the combustion temperature.
It is known to route exhaust gas from the exhaust passage leading from the combustion chamber back to the intake passage leading to the combustion chamber through which fuel and air for combustion are supplied. This system is known as an exhaust gas recirculation (EGR) system. Such a system generally comprises a passage leading from the exhaust passage to the intake passage, with the passage of exhaust gas therethrough regulated by a valve. This system is generally referred to as an "external" system since the exhaust gas is routed from the exhaust to the intake passage external of the flow path through the combustion chamber.
The external EGR system has the disadvantage that the exhaust gas which is routed from the exhaust passage to the intake passage is generally diluted with fresh air which fills the exhaust passage between exhaust strokes. Thus, the ratio of exhaust gas to the total amount of gas which is supplied to the intake air and fuel charge is reduced, as is the effectiveness of the exhaust gas recirculation in reducing harmful emissions.
In some situations, exhaust gas flowing from the combustion chamber into the exhaust passage is drawn backwardly into the combustion chamber at the same time the next air and fuel charge is being introduced. This situation generally arises when an intake valve opens the intake passage at the same time as the exhaust valve is open and exhaust is flowing into the exhaust passage.
During a portion of this period when both valves are open, or valve "overlap" as it is called, exhaust gas at high pressure is relieved through the exhaust passage. At the same time, however, the intake pressure within the intake passage leading to the combustion chamber is low, creating a suction effect. The next air and fuel charge naturally flows along the intake passage towards this low pressure area and into the combustion chamber. In addition, because the pressure in the exhaust passage is relatively high, some of the exhaust also flows back through the combustion chamber towards this low pressure area within the intake passage. Because this flow of exhaust gas is through the combustion chamber to the intake passage, it may be referred to as "internal" exhaust gas recirculation.
Unfortunately, this "internal" exhaust gas recirculation situation generally only occurs when the engine is running at low speed, and not at mid and high speeds where excessive heating results in the creation of the undesirable gases and the greater need for exhaust gas recirculation exists. In particular, as the engine speed increases, a throttle which controls the flow of air through the intake passage opens. As the throttle opens, the air pressure within the intake passage approaches atmospheric pressure. This relatively high pressure within the intake passage precludes the flow of exhaust in a direction generally other than directly out the exhaust passage, such that little exhaust gas recirculation occurs. In addition, exhaust gas recirculation at low engine speed may cause an over-dilution of the charge supplied to the combustion chamber in a manner which may cause the engine to misfire and run rough or die.
An improved exhaust gas recirculation system for an internal combustion engine is desired.